measure of synaptic strength, the authors measured the ratio of the AMPA receptor-mediated ESPC (AMPAR EPSC) to the NMDA receptor-mediated EPSC (NMDAR/EPSC); the so-called AMPAR/NMDAR ratio. This ratio was significantly increased for approximately a week following the injection of cocaine and several electrophysiological measures suggested that it was caused, at least in part, by an increase in the AMPAR EPSCs. A recent study reexamined this question by measuring unitary synaptic responses evoked by a highly localized glutamate source (2 photon photolysis of caged glutamate, Mameli et al. 2011 in press). The study concludes that both AMPAR transmission and NMDAR transmission are altered. A population of synapses shows strong rectification of the AMPAR-EPSC along with a decrease of the amplitude of the NMDAR-EPSC. Consistent with this conclusion, the magnitude of LTP at these synapses was reduced and that of LTD increased (Ungless et al., 2001; Dong et al., 2004). This transient enhancement of excitatory synaptic strength in VTA DA cells also occurs following administration of other addictive drugs, including morphine, nicotine, ethanol, and benzodiazepines, but not with non-addictive psychoactive substances such as fluoxetine or carbamazepine (Saal et al., 2003)2. Furthermore, prolonged self-administration of cocaine, unlike passive injections of cocaine or self-administration
